The preparation of crystalline microporous solids with pore openings greater than 1 nm has long been an important target for those interested in the adsorption, separation and catalytic conversion of large organic molecules. Aluminosilicate zeolites with such large pores, which could be used in the catalytic refining of the heavy fraction of crude oil, have not yet been prepared. The synthesis of metal organic frameworks with pore openings larger than 1 nm does not suffer from the same restrictions on the required tetrahedral geometry of framework components experienced by zeolites, and consequently structures with pore openings above 1 nm (and cages well into the mesoporous regime (>2 nm)) have already been prepared. One of the approaches by which extra large pore MOF solids can be prepared is via isoreticular synthesis, where ligands of increasing dimensions but the same coordination geometry can, under suitable conditions, form solids with the same framework topology but with different dimensions (and consequently pore sizes and pore window sizes). The IRMOF series of zinc carboxylates built from linear dicarboxylates is one example. (O. M. Yaghi, M. O'Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi, and J. Kim, Nature, 2003, 423, 705-714).
The presence of coordinatively unsaturated metal sites (CUS) in some MOFs is an important structural feature, particularly for considerations of chemisorption and catalysis. These are usually produced when solvent molecules coordinated to framework metal cations are removed by heating, leaving the framework intact but the metal cations without a full sphere of coordinating ligands. Examples of carboxylate MOFs that contain five-fold CUS upon desolvation include CPO-27 (Fe, Mn, Mg, Co and Ni). (P. D. C. Dietzel, R. E. Johnsen, R. Blom, H. Fjellvag, Chem. Eur.-J., 2008, 14, 2389-2397). Such coordinatively unsaturated metal sites are of much current interest as sites for adsorption of polar molecules or as Lewis acid sites in catalysis.
We have recently shown that it is possible to prepare large pore metal phosphonate MOFs which possess coordinatively unsaturated sites when dehydrated. The STA-12 series of divalent metal N,N′-piperazinebis(methylenephosphonate) materials, prepared with Mn, Fe, Co, and Ni in cation positions (FIG. 1), (J. A. Groves, S. R. Miller, S. J. Warrender, C. Mellot-Draznieks, P. Lightfoot and P. A. Wright Chem. Commun., 2006, 3305-3307; S. R. Miller, G. M. Pearce, P. A. Wright, F. Bonino, S. Chavan, S. Bordiga, I. Margiolaki, N. Guillou, G. Ferey, S. Bourrelly and P. L. Llewellyn, J. Am. Chem. Soc., 2008, 130, 15967-15981) has a pore size of 0.9 nm when dehydrated, and ca. 5 mmol g−1 of five-fold coordinated metal sites that are able to interact as Lewis acids with molecules such as CO and acetonitrile. However, the size of pore limits the potential application of such metal phosphonate MOFs.
It is amongst the objects of the present invention to obviate and/or mitigate at least one of the aforementioned disadvantages.